This invention relates to the gaseous reduction of particulate ores to metals in particulate form in a moving bed, vertical shaft reactor, and more particularly, to a method and apparatus for controlling the reduction of the ore and the cooling of the resulting metal particles. In the following description the process is illustratively described as applied to the reduction of iron ore to sponge iron. However, as the description proceeds it will be evident to those skilled in the art that the invention is also applicable to the treatment of ores other than iron ore.
In general, the production of sponge iron in a vertical shaft, moving bed reactor involves two principal steps, namely, reduction of the ore with a suitable hot reducing gas in a reduction zone of the reactor and then subsequent cooling of the resulting sponge iron with a gaseous coolant in a cooling zone of the reactor. The reducing gas is typically a gas largely composed of carbon monoxide and hydrogen at temperatures of the order of 850.degree. C. to 1100.degree. C., preferably 900.degree. C. to 1000.degree. C. The hot reducing gas is usually introduced at the bottom of the reduction zone and passed upwardly through the reactor to reduce the metal ore. In a number of previously proposed processes, cooling of the sponge iron is effected by passing a portion of the reducing gas at relatively low temperature upwardly through the cooling zone of the reactor whereby the reducing gas temperature is increased and the temperature of the sponge iron is reduced.
The sponge iron product is commonly used as the charge, or part of the charge, to an electric steel-making furnace. It has been found that when used for this purpose carburization of the sponge iron is desired. Carburization of the metal-bearing material in the reduction zone is achieved when a carbon-containing reducing gas is passed thereover. The product of the carburization process is primarily ferric carbide (Fe.sub.3 C) in which the carbon is in an especially suitable form for reaction with residual iron oxide (FeO) to produce sponge iron with a higher metallization.
In previously proposed processes this reaction between ferric carbide and residual amounts of iron oxide in the sponge iron has been accomplished in an electric steel-making furnace to which the sponge iron is charged after leaving the reactor. The amount of effort required to achieve a given degree of metallization increases relatively rapidly as the metallization approaches 100%. A metallization of up to about 85% can be obtained rather easily but in order to achieve higher metallizations up to say 95% to 98%, the residence time of the metal-bearing material in the reduction zone must be increased and higher gas temperatures must be used. As the residence time in a given size of reactor is increased, its productivity declines. Also since the effluent gas from the reduction zone must be cooled and de-watered before being reheated and recycled, the requirement that the inlet reducing gas be at a higher temperature results in greater heat losses and a lowered thermal economy. Moreover, if higher temperatures are used, there is a danger that the iron particles might sinter thereby interfering with the smooth flow of the particulate material through the reactor.
A need exists for a moving bed reactor system which will produce highly metallized and low-carbon sponge iron without a corresponding increase in the residence time through the reactor which would result in a reduction of the production rate. Sponge iron having a high metallization and low-carbon content can be used not only in the conventional electric steel-making furnace, but also for making steel in Cupola furnaces and as a raw material in the production of iron powders. The iron powder has a number of important applications including use in the fabrication of automotive and machinery parts, welding electrodes, parts for electric equipment and medicine.